parse-options: report uncorrupted multi-byte options

Because our command-line parser considers only one byte at the time
for short-options, we incorrectly report only the first byte when
multi-byte input was provided. This makes user-errors slightly
awkward to diagnose for instance under UTF-8 locale and non-English
keyboard layouts.

Report the whole argument-string when a non-ASCII short-option is
detected.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Improved-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'ef/non-ascii-parse-options-error-diag' into pu

* ef/non-ascii-parse-options-error-diag:
  parse-options: report uncorrupted multi-byte options
rebase -i: respect core.commentchar

Commit eff80a9 (Allow custom "comment char") introduced a custom comment
character for commit messages but did not teach git-rebase--interactive
to use it.

Change git-rebase--interactive to read core.commentchar and use its
value when generating commit messages and for the command list.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
[SQUASH??]
Merge branch 'jk/rebase-i-comment-char' into pu

* jk/rebase-i-comment-char:
  [SQUASH??]
  rebase -i: respect core.commentchar